(1) Field of the Invention
The present invention relates to a novel method of using differently charged polymers which are flocculants for clarifying aqueous solutions of a microorganism, particularly Escherichia coli ("E. coli") cells containing recombinant DNA which express a foreign protein. In particular, the present invention relates to a method which uses an anionic and a cationic polymer in sequence to flocculate the cells.
(2) Description of Related Art
Microorganisms in a culture or growth medium are typically extremely difficult to settle, thicken, concentrate and dewater which is referred to as "clarifying" in the industrial setting. The terms "microorganism" here particularly refers to bacteria, fungi and yeasts, which can be living or dead. The term "clarifying" designates dewatering, sedimentation, consolidation, flocculation, thickening, settling, concentrating or other separation activities which result in a clearer aqueous solution.
In order to clarify solutions containing microorganisms, a common procedure is to add cationic (positively charged) flocculants, which are polyelectrolytes, particularly high molecular weight derivatives of polyacrylamide. However, if the microorganism being treated does not contain sufficient surface negative charge, the treatment with cationic polymers is not always successful.
In some instances, the use of cationic polymers is improved with inorganic coagulants such as alum, ferric chloride or other salts. However, these materials can result in a less favorable sludge or floc due to a change in the pH or an increase in the amount of non-biodegradable and non-combustible material in the separated solids making them unattractive for landfilling or incineration. They can also cause increased corrosion to process equipment, can necessitate the handling of additional hazardous materials and can create greater floc volume.
Polymeric anionic and cationic flocculants are disclosed, for instance, in U.S. Pat. Re No. 31,900 to Halverson, U.S. Pat. No. 3,002,960 to Kolodny, U.S. Pat. Nos. 3,316,181 to Sakis, 3,686,109 to Aldrich, U.S. Pat. No. 3,692,673 to Hoke et al, U.S. Pat. No. 3,374,143 to Stephenson, U.S. Pat. No. 4,010,131 to Phillips et al, U.S. Pat. No. 4,451,628 to Damman, U.S. Pat. No. 4,565,635 to Du et al, U.S. Pat. Nos. 4,702,844 to Flesher et al, 4,693,830 to Thorton et al, 4,695,453 to Tuominen et al, U.S. Pat. No. 4,147,681 to Lim et al, and U.S. Pat. No. 4,770,803 to Forsberg. These polymers are derived from alpha-beta unsaturated monomers and the external charge depends on whether the pendent groups are positively (cationic) or negatively (anionic) charged. The problem has been that these flocculants have shown only limited utility in clarifying suspensions of microbial materials.